1. Field of the Invention
The present invention relates to a novel optical film comprising one or more polyarylates obtained from the condensation of one or more bifunctional phenolic units having specific substitutions with one or more bicarboxylic acid. More precisely, the present invention refers to a novel optical film, comprising one or more ortho-disubstituted bis(hydroxyphenyl)fluorene bisphenol polyarylates, having an excellent stability to ultraviolet and visible light, heat resistance, light transparency, high glass transition temperature (Tg) and very good mechanical properties.
2. Background of the Art
Optical films are well known in the art. Glass has been widely used for several optical applications, due to its excellent characteristics, such as transparency, a high transmittance in the visible light range and temperature resistance. Notwithstanding these benefits, due to its high weight and very high brittleness, the use of glass as a sublayer or support in optical applications may cause problems to the final product realization. In addition, because glass is not flexible, it cannot be used in continuous processing. This leads to a very low final productivity. For these reasons, it is undesirable to replace glass with transparent polymeric films, such as polyesters (e.g., polyethyleneterephthalate), polyacrylates (e.g., polymethylmethacrylate) or polycarbonate. Even if these polymeric materials have good handling properties, they have, however, the disadvantage of poor transmittance, limited heat resistance and a low glass transition temperature (Tg), whereby the employment of these polymers in optical applications has been very limited.
Polyesters of 9,9-bis-(4-hydroxyphenyl)-fluorene and isophthalic or terephthalic acid are known in the art. U.S. Pat. No. 3,546,165 describes such polyesters, which are thermally stable, but generally have a relatively low inherent viscosity and unsatisfactory mechanical properties. U.S. Pat. No. 4,387,209 describes polyesters made by reacting 9,9-bis-(4-hydroxyphenyl)-fluorene with at least one member of the group consisting of isophthalic or terephthalic acid and using an interfacial polymerization process. The polyester inherent viscosity strongly depends upon the monomer purity, and relatively small variations in purity of the diphenol monomer may cause large deviations in the inherent viscosity values. Example 2 of U.S. Pat. No. 4,401,803 describes the preparation of polyesters of 9,9-bis-(4-hydroxyphenyl)-fluorene and a 50:50 mixture of isophthalic and terephthalic acid chloride using an interfacial polymerization process. The polyester is precipitated with an acetone-methanol blend to produce a material with an inherent viscosity of 1.67 dl/g. U.S. Pat. No. 4,533,511 discloses a process for spinning fibers obtained from the polycondensation product of 9,9-bis-(4-hydroxyphenyl)-fluorene and a mixture of isophthalic acid chloride and terephthalic acid chloride. Dichloromethane is suggested as a spinning solvent and the solvent precipitant preferably is an aqueous lower alkanol.
U.S. Pat. No. 4,066,623 discloses that certain aromatic, halogenated polyesters, prepared by inverted interfacial polymerization methods provide a polymer having a low content of low molecular weight fractions which causes a low polymer resistance with respect to the solvent.
U.S. Pat. No. 4,967,306 discloses a 9,9-bis-(4-hydroxyphenyl)-fluorene/isophthalic and terephthalic acid polyester which contains a very low level of oligomeric material and has a tensile strength, elongation, chemical resistance, temperature stability, ultraviolet resistance and vacuum stability higher than the copolymers containing oligomeric species described in the art. Films containing small amounts of oligomer will “yellow” or degrade upon limited exposure to ultraviolet radiation.
The same polyarylate comprising 9,9-bis-(4-hydroxyphenyl)-fluorene and isophthalic and terephthalic acid is also described in JP patent application 57-192432. However, the resin color tends to the yellow upon exposure to UV radiation due to the diphenol component structure, thus making difficult the use of the resin in optical applications.
The resin obtained from the polyarylate is composed of 9,9-bis-(3-methyl-4-hydroxyphenyl)-fluorene and isophthalic acid as reported in Journal of Applied Polymer Science, Vol. 29, p. 35 to 43 (1984). The polymer composed merely of isophthalic acid, results to be too fragile and has insufficient abrasion resistance and film coating quality.
Japanese Patent Application No. 09-071,640 discloses a resin composed of (a) an aromatic dicarboxylic acid, (b) a specific amount of a substituted 9,9-bis-(4-hydroxyphenyl)-fluorene and (c) an aliphatic glycol; said resin is utilized in optical materials for its good transparency and heat resistance.
U.S. Pat. No. 4,810,771 discloses polyesters made of mono-ortho substituted bisphenols, and a blend of isophthalic and terephthalic acid.
EP Patent Application 943,640 describes a film prepared with polyarylates synthesized using bisphenolfluorenes mono- and bi-substituted in the ortho position with alkyl (C1–C4) groups. Such polyarylates have a better stability to ultraviolet radiation. 9,9-bis(3,5-dibromo-4-hydroxyphenyl)-fluorene bisphenol monomer derived polyarylates have been disclosed in PCT Patent Application No. WO 00-33,949 as gas-separation membranes. In U.S. Pat. No. 5,007,945, there is described a polyarylate class obtained from dicarboxylic acid chlorides and cardo bisphenols having halo-substituents on all ortho positions of the phenol groups, which polymer is used to separate one or more components of a gas mixture. Such patents describe gas-separation membranes, but do not mention optical films consisting of such polymers.
The resins, known in the art, obtained from polyarylates composed of 9,9-bis(4-hydroxyphenyl)-fluorene or its ortho mono-substituted homologous, even if they have good high temperature resistance characteristics and mechanical properties, are subjected to yellowing when exposed to light sources and can be therefore hardly used in optical applications.